Globe valves typically include a valve plug that is movable toward and away from a valve seat formed in a valve port between a fluid inlet and a fluid outlet. The movement of the valve plug is typically controlled by an actuator, such as a manual handle or some other type of remotely controlled actuation system.
With a typical globe valve, the flow of fluid through the valve is modulated by varying the clearance between the outside diameter of the valve plug and the inside diameter of the valve port. The clearance area between the outside diameter of the valve plug and the inside diameter of the valve port ranges from a minimum to a maximum value. Although a globe valve is able to provide precision flow control over almost the entire range of the plug stroke, limitations exist near the lower end of the flow value.
The limitations of a globe valve at low flow values are primarily due to the fact that the initial flow through a globe valve upon movement of the valve plug away from the valve port occurs around the entire circumferential surface between the valve plug and the valve port. Since both the valve port and the valve plug have diametral tolerances, as the valve plug strokes away from the valve port, the flow of liquid immediately bumps up to some minimum controllable value determined by the combined tolerance of the valve plug and the valve port. Therefore, currently available globe valves cannot provide the desired precision at low flow values and are thus not rated for this use.
The ratio of the full rated flow of a valve to the initial controllable flow is referred to as the valve's “rangeability”. The higher the rangeability value, the better the control system can operate to provide the desired fluid flow. Typically, valves in a family designed for low flow rates have the lowest rangeability. When utilizing such a valve, the control system has difficulty in achieving accuracy and stability requirements over the required flow rates, especially at low flow rates.
Accordingly, there is currently a need for a globe valve that provides precise volumetric control at low flow rates and higher rangeability. Additionally, there is a need for a globe valve that substantially eliminates the “bump” at the beginning of fluid flow through the valve to increase the effective rangeability of the valve.